NK cell are the primary effectors of the innate immune response against infectious pathogens and malignant transformations through their efficient cytolytic activity. Unlike T and B cells, NK do not recognize antigens in the context of classical major histocompatibility complex (MHC) but they lyse tumor cells without specific antigen recognition. Although several studies have shown that the ability of NK cells to lyse the target is dependent on the expression of several inhibitory or activating receptors, the molecular mechanism that are involved in the target cell susceptibility to NK cells are still not well understood. We hypothesized that the use of a genetic screening strategy would identify novel pathways that modulate tumor cell susceptibility to the immune system. To test this hypothesis, we developed a cell-cell interaction shRNA based screening using IM- 9 tumor cells as target and NK cells as effectors. Among the genes that modulated target cell susceptibility to NK activity we found 2 members of the Jak family (Jak1 and Jak2). Independent experiments confirmed that while target cells knock-down for Jak1 and Jak2 were more susceptible to NK cell activity no effect was noticed with the other 2 members of the Jak family (Jak3 and TYK2). Preliminary gene expression profile analysis showed that target cells knock down for Jak1 significantly modulated the expression of genes in the TRAIL/FASL pathway. These pathways are known to induce target apoptosis after NK cells engagement. In the current study we propose: 1) a series of experiments to better understand how Jak1 and Jak2 can modulate tumor susceptibility through the TRAIL/FASL pathway. 2) Analyze how Jak genes modulate susceptibility of a series of primary tumor cell subpopulation to NK or T cell activity. 3) Start to define possible downstream molecules that are also involved in the induction of tumor sensitivity to NK cells. NK cells represent one of the main effectors of immune surveillance in the body, playing an important role in patrolling the bloodstream, spleen, bone marrow and other lymphoid organs to eliminate virus infected and tumor-transformed cells. A better understanding of how NK cells recognize their targets will help to improve clinical outcome in cancer patients and will lead to further advances in the clinical application of NK cell therapy to prevent and effectively treat cancer. PUBLIC HEALTH RELEVANCE: NK cell are the primary effectors of the innate immune response against infections pathogens and malignant transformations through their efficient cytolytic activity. Although several studies have shown that the ability of NK cells to lyse the target is dependent on the expression of several inhibitory or activating receptors, the molecular mechanism that regulate the target cell susceptibility to NK cells are still not well understood. To identify new genes that can induce tumor cell susceptibility to the innate immune system we developed a cell-cell interaction shRNA based screening. Multiple independent shRNAs, targeting more than 1,000 genes, were transduced in a tumor target cell line in an high-throughput way and tested for their susceptibility to NK cells. Two members of the Jak family genes were found to significantly modulate tumor susceptibility to NK cells. In the current study we propose to investigate the involvement of the Jak genes in the induction of tumor cell susceptibility to the NK cell activity. We anticipate that our studies will: 1) better determine the mechanisms on how Jak genes modulate tumor susceptibility to NK cell activity. 2) Analyze how Jak genes modulate susceptibility of a series of primary tumor cell subpopulation to NK or T cell activity. 3) Start to define possible downstream molecules that are also involved in the induction of tumor sensitivity to NK cells.